I had an idea where there's an SCP that, if triggered by certain phrases, words, actions, or ideas relating to or on the topic of death/pain, would go through three stages of anger, the third being one in which it quickly moves to any nearby human, pulls out the spine, and snaps it.

I really want to know how I can make this idea more interesting because I know that it's just a "quick moving, boom, you dead" kind of a thing..

This feels a lot like a SCP-173 kinda thing, and an analogy could work here.

Blinking is to 173 as words/phrases/actions about death/pain are to your SCP idea.

I'd change what triggers the stages of anger. Perhaps there's some deeper narrative as to why your SCP acts like that, or perhaps you could change its reaction altogether. You need to step away from the analogy I stated above, as that will help make your idea more original. I hope this helps.

Even more than the 173 comparison, the issue with this is that it's boring. It is just two of the most common SCP pitfalls rolled into one: a compulsion effect and a "thing that kills you."

Compulsion effects are inherently cheap writing due to forcing characters to act in a certain manner rather than writing situations that lead interesting decisions to be made naturally. They can work as minor plot points in a larger article, but if they form the basis then they virtually never succeed. As to the latter, simple anomalies that do little but cause death are seen as cheap attempts at shock value without thematic depth.

I recommend either drastically overhauling this concept or scrapping it outright.